1. Field of the Invention
This invention relates to a method for the production of a transparent conductive ZnO film and more particularly to a method for the production of a transparent conductive ZnO film which can be formed at a relatively low temperature and whose surface form can be controlled as desired by selecting the film-forming conditions.
2. Description of the Prior Art
As the transparent conductive film which finds utility as in touch switches, liquid crystal display elements, and EL (electroluminescent) devices, an indium oxide-tin oxide type transparent conductive film known popularly as an "ITO film" has been generally adopted to date. Since the indium, one of the raw materials for this ITO film, is a rare metal, the ITO film is expensive by reason of acquisition as a resource. Further, since the ITO film requires a treatment at elevated temperatures in the range of from 250.degree. C. to 600.degree. C., it entails the disadvantage that the production consumes a large amount of energy and proves to be expensive.
As the transparent conductive film used in the thin-film solar cell, SnO.sub.2 has been finding popular acceptance to date. However, the SnO.sub.2 film has a problem of unduly large energy consumption because the film-forming temperature of this compound is so high as to fall typically in the neighborhood of 500.degree. C. Further, the thin film of SnO.sub.2 has the disadvantage that it finds only conspicuously limited utility in view of the heat resistance of the substrate. When a thin film of SnO.sub.2 is to be formed on the surface of a glass substrate, for example, a film of SiO.sub.2 is preparatorily formed as an undercoat on the surface and then the film of SnO.sub.2 is superposed on the undercoat. Thus, the superposition of the thin film of SnO.sub.2 turns out to be complicate in point of treatment.
Recently, the fitness of ZnO as a transparent electrode and an unreflective film for solar cells of amorphous silicon and CuInSe.sub.2 has come to find popular recognition. As measures to effect production of a transparent conductive ZnO film, the spattering method, the ion plating method, and the vacuum deposition method have been generally known in the art. It has been also known that the film can be produced by the chemical vapor deposition process or other film-forming technique, as disclosed in the Japanese Patent Application laid open to public inspection (KOKAI) No. 154,411/1987.
Since the conventional film-forming techniques mentioned above except for the spattering method require elevated temperatures in the formation of a film, in the manufacture of a device, the possibility of the performance of the produced device being impaired by the elevated temperature used during the formation of a film and the problem of the film formation at an elevated temperature mentioned above must be taken into due consideration. On the other hand, when the spattering method is adopted for the formation of the film, the control of the form of the film surface is not attained, depending on certain conditions of the operation. Specifically, the spattering method does not easily confer a textured form (rugosity or unevenness) upon a film surface. When the thin film of ZnO on the surface of a sensitive part as in a solar cell, for example, has a textured form, the textured form can be expected to produce an effect of scattering light and, in consequence of the resultant addition to the length of a light path, to enhance the performance of the device. In the circumstances, the desirability of developing a method for forming a thin film of ZnO possessed of a textured surface form has been finding growing recognition.